Optical quantum memory designed

By
Eric Smalley,
Technology Research NewsSince a landmark 2001 paper showed that
quantum computers could be built using ordinary optical equipment like
mirrors and beam splitters, researchers have been working on practical
implementations.

Most designs call for using atoms or electrons as quantum bits,
or qubits, because these types of quantum particles interact more readily
than photons. Properties of the quantum particles that make up qubits
can represent the 1s and 0s of digital information. A photon, for instance,
can be polarized in two perpendicular directions, and one direction can
represent 1 and the other 0.

But closely controlling small numbers of atoms or electrons requires
complicated laboratory equipment, and these designs also require some
means of transferring quantum information to photons in order to transmit
information. Linear optical quantum computing overcomes these problems
but requires a reliable method of briefly storing photonic qubits -- a
major challenge considering the fleeting nature of photons.

Researchers at NASA's Jet Propulsion Laboratory have designed
an optical quantum memory device capable of storing photonic qubits for
use in all-optical quantum computers and quantum communications networks.

The researchers' quantum transponder could be used to make quantum
repeaters that would extend the distances covered by emerging quantum
cryptography systems. Quantum cryptography provides theoretically perfect
security because eavesdroppers unavoidably disturb quantum information
in detectable ways.

The transponder could also eventually be used in all aspects of
quantum information processing, said Robert Gingrich, now a researcher
at the California Institute of Technology. "We are aiming to design and
ultimately build a complete quantum Internet using only linear optical
elements," he said. "The quantum computers at each node and the quantum
communication lines that connect them [would] all [be] made out of these
simple resources."

The scheme calls for encoding pairs of quantum bits in sets of
four photons in such a way that the two qubits can be read even if one
of the four photons is lost. Qubits would be sent into a fiber loop and
a simple quantum computer would correct for errors caused by photons being
absorbed by the fiber. "Think of the fiber loop as a NASCAR race track
and the qubit... as a race car. The quantum transponder or error-correcting
box is the pit stop," said Gingrich. "The qubit degrades as it goes around
the loop and so periodically it must be fixed at the pit stop."

The transponder includes a device that would generate single photons
to replace lost photons.

The device could be used to extend quantum communications, including
quantum cryptography, by linking multiple transponders in series. The
transponders would allow quantum information to be transmitted over longer
distances by simply correcting errors, unlike previous designs for quantum
repeaters, which extend transmission distances by manipulating the weird
quantum state of entanglement in pairs of atoms or subatomic particles.
When particles are entangled, properties like polarization change in lockstep
regardless of the distance between the particles. Pairs of entangled particles
can be used to transmit information, but entanglement is a relatively
fragile state.

Building practical optical quantum memory devices will require
better photon detectors and devices that can reliably emit single photons
on demand, said Gingrich.

The quantum transponder could be used in practical applications
in five years, said Gingrich.

Gingrich's research colleagues were Pieter Kok, Hwang Lee, Farrokh
Vatan and Jonathan Dowling. They published the research in the November
21, 2003 issue of Physical Review Letters. The research was funded
by the National Security Agency (NSA), the Defense Advanced Research Projects
Agency (DARPA), the National Reconnaissance Office, the Office of Naval
Research (ONR), the National Research Council and NASA.